1. Field of the Invention
The present invention relates to a method and apparatus for the control of movement of a sheet, so as to properly register it with a prescribed area on a moving member such as a drum or belt. Such drums and belts are used in xerographic copiers, printers, and the like. More particularly, the present invention relates to a method and apparatus for moving a copy sheet in a manner so as to align and synchronize movement of the sheet with an area of a member containing an image such as a toned electrostatic image associated with xerographic apparatus.
2. Description of the Prior Art
Xerographic and electrostatic copiers and printers impart an image to a copy sheet which is extracted from a supply and is fed to a drum or belt containing the image which is to be transferred to the sheet. Generally, this transfer is done while both the drum or belt and the sheet are moving to enhance the productivity of the apparatus. In order to assure that the image is satisfactorily aligned with the sheet, it is necessary to register the sheet with the drum or belt containing the image prior to the image transfer. When multiple images are to be registered to form a single image, such as when a series of mono-color images are superposed to form a full color image, even better registration is required. A variety of sheet registration devices are known in the prior art including mechanical gates which engage an edge of a sheet to hold it until the appropriate time and then release it for engagement with the image-bearing member. Typically, the gate is actuated by mechanical cams and switches, or the like, with the copy sheet, upon release, being driven by pinch rollers into registry with the image area as it passes the transfer zone. In such an arrangement, the pinch roller velocity is controlled to move the copy sheet at the same speed as the moveable member containing the image.
Some registration devices for copiers employ digital circuitry to monitor the image position and to control operation of the mechanical release gates and pinch roller drives. One example is shown in the IBM TECHNICAL DISCLOSURE BULLETIN of May 1980 (Volume 22, No. 12), in the article entitled "Servo-Controlled Paper Gate" by J. L. Cochran and J. A. Valent at pages 5,268-5,269. Digital circuitry shown in this article monitors the photoconductor image frame location and controls actuation of a mechanical copy sheet gate as well as the DC motor drive for the copy sheet to bring the speed of the copy sheet up to the speed of the photoconductor as it engages the image panel.
Another application of digital controls for copy sheet alignment is shown in U.S. Pat. No. 4,310,236 by J. L. Connin, filed Oct. 12, 1979, wherein stepper motors are used to position mechanical gates so that the copy sheets are fed with a skew that conforms to the original document skew as it was imaged onto a photoconductor belt. A logic and control unit monitors the photoconductor image location as it moves and digitally compensates for the skew as measured by sensors at the original document when it was imaged onto the photoconductor.
It is also known to utilize stepper motors to control the movement and positioning of original documents presented for scanning by a copier. For example, U.S. Pat. No. 3,888,579 by V. Rodek et al., filed Jan. 31, 1974, shows a combination of stepper motors and rollers, sheet detectors and controls that function to release the original documents so that the document image correlates to a predetermined image area on the photoconductor. It is thus known to monitor the photoconductor image zone location and to sense with a photocell the movement of an original document to control release of that document so that it passes a scanning window with a velocity compatible with the photoconductor velocity. And further, with the proper timing, it is possible to place the original document image in the predetermined image zone on the moving photoconductor.
However, such prior art registration systems can cause slippage or scrubbing of the copy sheet by the drive members or pinch rollers while it is waiting release by the gate to the image transfer area, thus creating an area thereon which can adversely affect the image. Moreover, the edge of the sheet is often indented or buckled by the gate, particularly when light-weight paper is being used, which further adversely impacts on the registration when the gates are released.
More significantly, prior art registration devices generally require very close tolerances and careful alignment of the detectors with the other components of the image transfer apparatus. As a result, even though the prior art registration systems may operate satisfactorily, they require lengthy, costly setup when the apparatus is assembled, plus careful maintenance over the life of the apparatus. All of this contributes to increased costs and maintenance over the life of the apparatus.
Accordingly, a registration system that is easily set up and requires a minimum of maintenance and which reliably registers a sheet with a moving member for the transference of an image would be extremely desirable.